The preferred embodiment concerns a method, a device and a computer program to correct a registration error in a printing process that is due to deformation of the recording medium.
In printing processes in which a recording medium is subjected to a temperature treatment, the recording medium normally deforms. For example, if the recording medium is printed repeatedly in the same print group or by different print groups in the course of the printing process and a heat treatment takes place between the printing processes, the problem frequently exists that—due to the shrunk recording medium—the image to be printed appears larger in the second printing process than the image printed in the first printing process. Registration errors hereby arise that, for example, are disruptive if the recording medium is printed on both sides and the back-side printing region appears larger (due to the shrinkage effect) than the front-side printing region. In particular in book production, such registration errors are disruptive and incompatible with a high demand for print quality. In color printing in which color separations are printed successively in different print groups and a corresponding shrinkage effect occurs between the print groups, the color quality can also suffer from the fact that the color separations are not printed in register.
In many printing processes paper is used as a recording medium. The effect that the moisture naturally contained in paper escapes from the paper due to a heat treatment and that a shrinking of the paper occurs both in the direction of the paper fibers and transverse to the paper fibers can be of greater or lesser severity. For example, the severity thereby depends on the fiber direction, on the fiber or paper type, the storage conditions of the paper and additional conditions in the printing process, for example the environment humidity, the temperature etc.
Such a shrinking effect can occur in any type of printing process, in particular in electrographic printing processes and inkjet printing processes. In printing processes that operate with water-based inks, the shrinking effect of the paper can be particularly strong due to the strong moisture absorption of paper in the course of printing with the ink and the subsequent drying process for the ink.
Paper is a hygroscopic material that emits water vapor upon heating and stores water again upon cooling. The heat-dependent shrinkage can typically amount to 1 to 2% in printing processes of the aforementioned type, but also more or less. Given a paper width of 20″ and a shrinkage factor of 1%, the shrinkage in the direction of the paper width amounts to approximately 5 millimeters, for example. Paper can also expand in the course of the cooling and storage of water.
In inkjet printing systems in which a paper web is printed in two print groups and the paper or the ink is dried by means of heat in a drying process after the first printing process, the paper is significantly dehumidified, whereby it shrinks. As described above, this leads to register deviations between the information printed by the two print groups. In such printing apparatuses this additionally leads to problems when they are designed print the complete paper width, i.e. when what is known as an edge-to-edge printing takes place. The shrinking of the paper in the region of the second print head then leads to the situation that the information at the outermost image edge can no longer be printed on paper; rather, the ink provided for this is sprayed, unused, past the paper. Regions of the print group—for example bearing parts, contact pressure rollers etc.—can thereby additionally be soiled, whereby cleaning processes are necessary. Furthermore, a print product that is processed later and that runs through this print group can thereby be soiled so badly that it must be discarded as spoilage. A significant danger is also that important print data of the border region on the print product are not printed, and therefore the respective printed document is incomplete.
The aforementioned problems appear particularly disruptively if the printing process occurs by means of a print head in which a discrete number of write elements produce the printing process, for example light-emitting diodes arranged in a row or matrix to expose an electrophotographic element or inkjet nozzles arranged in rows and/or columns.
A method for registration correction with paper shrinkage is known from DE 10 2007 040 402 A1, in which a print element row in the second printing process is arranged rotated by an angle relative to a print element row of a first printing process with regard to a predetermined axis of the paper movement direction, such that the distance between the pixels (image elements) generated with the print element rows is reduced. In the second printing process an image that is shrunk corresponding to the paper shrinkage is thereby written to the paper web. What is disadvantageous in such a method is that, for example, mechanical adjustments to the print head that correspond to the degree of shrinkage are necessary, which adjustments are relatively complicated.
Furthermore, in DE 10 2007 040 402 A1 it is proposed for a duplex printing process to shift print frames of the print images of the front side and back side counter to one another so that their central axes coincide. It can thereby be produced that the front and back sides overlap centrally and the registration errors at the edges of the print images distribute symmetrically on both sides, i.e. are essentially centered. Given a printing process that starts per line at one lateral edge of the recording medium, this furthermore has the consequence that the registration errors at the row end are halved. However, a complete compensation of the registration error cannot be brought about by this. For large print widths (for example of 30″) and a degree of shrinkage of 1%, this still leads to a double-sided registration error of approximately 4 millimeters, respectively at the edges along the axis of shrinkage, which is unacceptable in many printing applications.
A method to correct paper shrinkage that starts with the generation of a bitmap is known from WO 2005/031470 A1. To generate an image for the first printing process or for the second printing process, the number of pixels to be considered in the printing is modified corresponding to the shrinkage to be expected in the direction of one axis. For this it can be provided to enlarge the rastered and dithered image for the first printing process by inserting pixels corresponding to the degree of shrinkage, or to reduce the image for the second printing process by deleting pixels corresponding to the degree of shrinkage. What is disadvantageous in the method described there is that the insertion or omission of entire pixels in the bitmap can lead to image distortions or artifacts.
FIG. 2 schematically shows the method described in WO 2005/031470 A1. A bitmap 1 in which a binary image 2 is stored is subjected to a shrinkage compensation process 3. The process takes place along an image processing axis BA in which an image column 6 is deleted, whereby the image 2 is shrunk to an image 7. The shrinkage factor 4 that is thereby achieved or enters into the process corresponds to a shrinkage factor of the recording medium 11 in the printing process along its shrinkage axis SA. The adjacent column 6a still remains in the target image 7 due to the deletion. However, the remaining pixels in this region no longer form squares; rather, they are only rectangles, meaning that an artifact exists in the target image 7. The unwritten border region 9 to the right of target image 7 is, however, larger by one column than the unwritten border region 8 of the source image, whereupon the image shrinkage that is produced is apparent.
The print head 10 that prints the paper web 11 is then controlled with the data of the target image 7. The shrinkage axis SA corresponds to the paper web 11 of the image processing axis BA.
An electronically assisted method for the correction of registration errors in electrographic color printers is known from U.S. Pat. No. 4,721,969 A, in which registration errors of the color separations are determined using registration markings that are detected with CCD sensors.
The aforementioned publications are herewith incorporated by reference into the present Specification.